Assessment of heterogeneities of the shallow crust from geophysical logging in the Koyna-Warna region, Deccan Volcanic Province, India

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Analysis and comparison of geophysical logs from seven exploratory boreholes of 1–1.5 km depth in the Koyna-Warna region of India illustrate variations in physical properties and structural characteristics of the basalts and underlying granite-gneiss basement in and around the seismogenic zone. In the north, south of the Koyna reservoir, densities are higher, gamma values are lower, fractures dip at high angles with low fracture permeabilities. To the south, around the Warna reservoir, densities are lower, gamma values higher, and resistivity values are low. In the southernmost borehole at Khadi, basement is highly fractured, and there are significant amounts of water outflow associated with the fracture system. The fracture density increases to the south, with fracture dips ranging from medium to low angles with permeability values two orders of magnitude higher than in the northern boreholes. These variations in physical and structural properties resulting in a highly heterogeneous crust, reflect the cumulative effect of episodes of tectonic activity over millions of years that this region has undergone.